Oscillator grid-leak circuit



Patented June 26, 1951 OSCILLATOR GRID-LEAK CIRCUIT Sheldon I. Rambo and Bruce Boyd, Baltimore,

Md., assignors to Westinghouse Electric Corporation, East Pittsburgh, Pa., a corporation of Pennsylvania Application July 14, 1949, Serial No. 104,736 12 Claims. (01. 250-36) Our invention relates to electronic tube circuits for generating high frequency oscillations, and in particular, it relates to a novel type of grid-leak resistor for employment with such tubes. It is particularly valuable for oscillation generators supplying power to high frequency induction heating systems.

The loads on high frequency oscillators employed for industrial heating applications are likely to vary widely from tim to time and as a result, there is a strong tendency for the oscillator grid currents to change through excessive ranges.

One object of our invention is to provide a type of grid-leak which will tend to minimize the variations in grid current due to variations of load over a wide range for oscillation generators of the electron tube type.

Another object of our invention is to provide a novel type of circuit connection for high frequency electron tube oscillation generators.

Still another object of our invention is to provide an oscillation generator circuit of a novel type for supplying power to high frequency heating loads.

Still another object of our invention is to provide a novel type of grid-leak for high frequency oscillation generators of the electron tube type.

Other objects of our invention will become apparent upon reading th following description taken in connection with the drawing, in which the single figure is a diagrammatic showing of a circuit connection for electron tubes which em bodies the principles of our invention.

In the case of oscillation generators of .the electron tube type, having coupled grid and plate circuits, the grid or control electrode is usually coupled to the cathode through a capacitor shunted by a grid-leak resistor, which arrangement automatically provides the control electrode with a suitable negative bias to produce eflicient operation of the tube as an oscillator. However, where such an. oscillation generator is called upon to supply loads of a wide range of electrical characteristics, such for example as high frequency industrial heating loads are likely to be, the required value of the grid-leak resistor changes from time to time with the condition of the load supplied. It is obviously undesirable to hav to manually regulate, from time to time, the value of the grid-leak resistor to meet these requirements.

In accordance with our invention, we avoid this necessity for manual variation of a grid-leak resistor by employing for that element a resistor having the property of passing a current which undergoes a relatively small variation for the considerable variation of its terminal voltage. Specifically, we find it possible to employ grid-leak resistors comprising tungsten lamps, inasmuch as such lamps have a volt ampere characteristic in which the current change is relatively small for a relatively larg variation of terminal voltage over a considerable range of the latter below their normal rated voltage. However, this property of relative invariability of current with voltage change is met with only in the range of voltages above a certain value, and for lower voltage, the per cent current change is almost as great as the per cent voltage change, as is the case in an ordinary resistor.

In" particular, the value of the resistance of such tungsten lamps over lower range of the terminal voltage far below their rated operation is too small to meet the requirement for efficient and certain starting of the oscillation generator after voltage is first applied to it. That is to say, the voltage on the grid-leak resistor is low at the time power is first applied to the oscillation generator tube and'because of the low resistance of the tungsten lamp grid resistors at these low voltages, the tube may not start to produce oscillations.

, In order to avoid the last-mentioned difliculty, We provide the circuit shown in the drawing by means of which the tungsten lamp filaments are heated to around their normal operating value from a separate source of current in the first instants after applying voltage to the oscillator tube, whereby the grid-leak resistor will have a sufiiciently higher resistance to initiate oscillations. Once oscillations begin, the heating current from the separate source is out off and the tungsten grid-leak is permitted to function with its ordinary electrical characteristics.

, It would, of course, be possible to supply the deficiency of resistance in the tungsten lamps at the instant of applying power to the oscillation generator and tube by connecting a sufiicient resistance of the ordinary type in series with the tungsten lamps. However, this would prevent obtaining the full value of the constant current characteristic of the tungsten lamps alone, so that the arrangement which we have provided for preheating of the tungsten filaments before initiation of oscillations eliminates an undesirable feature which would be present were such an ancillary control resistor employed.

With the foregoing principles in mind, attention may be directed to the drawing in which an electron tube l of conventional type provided with an anode, a cathode and a control electrode is shown connected through the inductanc 2 of a tank circuit also including a capacitor 3 to the positive terminal of a source of direct current (not shown) having its negative terminal grounded. The cathode of the tube l is connected to ground .and is also connected through a grid con denser 4 and an inductor 5 to the control electrode of the tube l. The inductor 5 is inductively coupled to the inductor 2 of the tank circuit in a way well known in the art to cause the system to operate as a self-excited oscillator. The positive terminal of the direct current source is connected to ground through a by-pass capacitor 6. A load I which may, for example, be a high-frequency heating load is coupled through a winding 8 with the tank circuit inductor 2.

The grid capacitor 4 has its terminal shunted by a resistor 5 comprising one or more tungsten lamps. The lamps Q are so designed that the discharge current of the capacitor 4, when the control electrode of the tube 1 has its desired negative bias, heats the filaments of the tube 9 sufficiently so that the latter is operating on a portion of the ampere curve in which the voltage changes very rapidly with variations of lamp current. The lamp 9 has shunted across its terminals a circuit comprising a pair of separable contacts in series with either winding, e. g. the secondary winding ll, of a transformer having a primary winding [2 connected through a proper switch 13 to an alternating current source which may be of an ordinary commercial frequency. The separable contacts [0 are those of a normally closed relay Kl having an energizing winding connected to derive current from the tank circuit 23 whenever oscillations are being produced by the oscillation generator tube I. Thus, the energizing winding of the relay KI is connected in series with a rectifier l4 across the terminals of a capacitor which is connected through a second capacitor I6 between a point on the tank circuit 23 and ground.

The mode of operation of the above-described arrangement we believe to be as follows. When the apparatus is disconnected from its power sources, the contacts ill of the relay Kl are closed, since the energizing winding of that relay is drawing no current from the unenergized tank circuit 2--3. To initiate operation of the system, the switch it is closed at the same time that voltage is supplied from the positive terminal to the circuit of tube l. Current will, therefore, flow from the secondary winding 1 I through the closed contacts it and heat the resistor 9 to its normal temperature at which it has a suiiiciently high resistance to close the tube l to readily produce high frequency oscillations. Current will, accordingly, flow through the tube l' generating oscillations in the tank circuit 2-3 and causing current thus to flow through, the capacitor l6 and rectifier it and energizing winding of the relay Kl. The latter will immediately open the contacts H3, cutting off current flow through the resistor 9 from the secondary winding, but the resistor will be operated at whatever temperature its electrical characteristics dictate by the current flowing from the control electrode circuit of tube I.

The system will, accordingly, then operate and continue to operate as long as oscillations are present in the tank circuit 2-3, with the secondary winding ll supplying no current through inductively separated contacts H! to resistor 9. However, whenever oscillations cease to be generated in the tank circuit 2-3, the energizing winding of relay Kl will cease to carry current and the separable currents ill will reclose to supply current to the resistor 9 from windin H, thereby maintaining the resistor 9 at its desired high resistance.

When, however, it is desired to completely remove the system from its operating condition, the switch [3 may be open to avoid supplying unnecessary heat to the resistor 9.

It should be noted that the resistor 9 is maintained in its desired operating condition with the system above described, notwithstanding any brief failure of the generator I to produce oscillations, and that should the generator 1 resume the production of oscillations after any unintended shutdown, the resistor 9 has been maintained at the value necessary to immediately cause the desired operation of the oscillation generator I. It will also be obvious that our arrangement for providing a grid resistor having the desirable voltage characteristics described in connection with the tungsten lamps 9 may be applicable in the case of electron tubes for many other purposes; for example, it will be useful in other well known oscillation generator circuits, such as the Hartley circuit and Colpitts circuit.

Since numerous changes may be made in the above-described construction, and different embodiments of the invention may be made without departing from the spirit and. scope thereof, it is intended that all matter contained in the foregoing description or shown in the accompanying drawing shall be interpreted as illustrative and not in a limiting sense.

We claim as our invention:

1. In combination with an electric discharge tube having a pair of principal electrodes and a control electrode, a source of voltage supplying current flow between said principal electrodes, a tank circuit in coupled relation with the circuit between said control electrode and said principal electrodes, the circuit of said control electrode being provided with a capacitor shunted by a grid-leak comprising a resistor having a volt ampere characteristic in which the percentage change of current is small compared with the percentage change of voltage in a predetermined range, and an ancillary circuit connected to cause current flow through said resistor additional to the leakage current from said capacitor.

2. The arrangement described in claim 1, in which means are provided to connect said ancillary source to said grid-leak resistor when the power in said tank circuit is below a predetermined value and to disconnect said ancillary source from said grid resistor when the power in said tank circuit is above a predetermined value.

3. In combination with an electron tube having a cathode, an anode and a control electrode, a capacitor connected between said control electrode and said cathode and a by-pass resistor shunting said capacitor, said by-pass resistor having a volt ampere characteristic in which the percentage change of current with voltage is relatively small over a predetermined range, an ancillary current source for supplying current to said by-pass resistor when the power flowing in the circuit of said tube is below a predetermined value, and means for interrupting current flow from said ancillary source to said by-pass resistor when the power flowing through said tube is above a predetermined value.

4. The arrangement described in claim 3, in which the by-pass resistor is a material having a strong positive temperature-resistance coefiicient.

5. The arrangement called for by claim 3, in which the by-pass resistor is an incandescent lamp having a tungsten filament.

6. The arrangement described in claim 1, in which the by-pass resistor is a material having a strong positive temperature-resistance coefiicient.

7. The arrangement described in claim 2, in which the resistor is a material having a strong positive temperature-resistance coeflicient.

8. The arrangement described in claim 1, in which the resistor is an incandescent lamp having a tungsten filament.

9. The arrangement described in claim 2', in which the resistor is an incandescent lamp having a tungsten filament.

10. An oscillation generator for high frequency heating, comprising an electron tube having a pair of principal electrodes and a control electrode, a source of voltage for supplying current flow between said principal electrodes through a tank circuit comprising an inductor shunted by a capacitor, means for coupling a high frequency heating load to said inductor, a winding coupled to said tank circuit inductor and connected in series with a capacitor between said control electrode and one said principal electrode, a bypass resistor shunting the last-mentioned capacitor, said by-pass resistor being of a type in which the percentage change of current with percentage change of voltage is comparatively low over a substantial range, an ancillary current source connected to supply current to said bypass resistor through a pair of normally closed separable contacts, and a. motor winding for moving said separable contacts, said motor winding being connected in series with a rectifier which is energized in accordance with the current fiowing in said tank circuit.

11. The arrangement specified in claim 10 in which the by-pass resistor is an incandescent lamp containing a filament which has a strongly positive temperature-resistance coefficient.

12. The arrangement specified in claim 10 in which the by-pass resistor is a tungsten lamp.

SHELDON I. RAMBO. BRUCE BOYD.

REFERENCES CITED The following references are of record in the Brown May 20, 1947 

